+static int disable_mtrr_trim;
+
+static int __init disable_mtrr_trim_setup(char *str)
+{
+ disable_mtrr_trim = 1;
+ return 0;
+}
+early_param("disable_mtrr_trim", disable_mtrr_trim_setup);
+
+/*
+ * Newer AMD K8s and later CPUs have a special magic MSR way to force WB
+ * for memory >4GB. Check for that here.
+ * Note this won't check if the MTRRs < 4GB where the magic bit doesn't
+ * apply to are wrong, but so far we don't know of any such case in the wild.
+ */
+#define Tom2Enabled (1U << 21)
+#define Tom2ForceMemTypeWB (1U << 22)
+
+int __init amd_special_default_mtrr(void)
+{
+ u32 l, h;
+
+ if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
+ return 0;
+ if (boot_cpu_data.x86 < 0xf || boot_cpu_data.x86 > 0x11)
+ return 0;
+ /* In case some hypervisor doesn't pass SYSCFG through */
+ if (rdmsr_safe(MSR_K8_SYSCFG, &l, &h) < 0)
+ return 0;
+ /*
+ * Memory between 4GB and top of mem is forced WB by this magic bit.
+ * Reserved before K8RevF, but should be zero there.
+ */
+ if ((l & (Tom2Enabled | Tom2ForceMemTypeWB)) ==
+ (Tom2Enabled | Tom2ForceMemTypeWB))
+ return 1;
+ return 0;
+}
+
+/**
+ * mtrr_trim_uncached_memory - trim RAM not covered by MTRRs
+ * @end_pfn: ending page frame number
+ *
+ * Some buggy BIOSes don't setup the MTRRs properly for systems with certain
+ * memory configurations. This routine checks that the highest MTRR matches
+ * the end of memory, to make sure the MTRRs having a write back type cover
+ * all of the memory the kernel is intending to use. If not, it'll trim any
+ * memory off the end by adjusting end_pfn, removing it from the kernel's
+ * allocation pools, warning the user with an obnoxious message.
+ */
+int __init mtrr_trim_uncached_memory(unsigned long end_pfn)
+{
+ unsigned long i, base, size, highest_pfn = 0, def, dummy;
+ mtrr_type type;
+ u64 trim_start, trim_size;
+
+ /*
+ * Make sure we only trim uncachable memory on machines that
+ * support the Intel MTRR architecture:
+ */
+ if (!is_cpu(INTEL) || disable_mtrr_trim)
+ return 0;
+ rdmsr(MTRRdefType_MSR, def, dummy);
+ def &= 0xff;
+ if (def != MTRR_TYPE_UNCACHABLE)
+ return 0;
+
+ if (amd_special_default_mtrr())
+ return 0;
+
+ /* Find highest cached pfn */
+ for (i = 0; i < num_var_ranges; i++) {
+ mtrr_if->get(i, &base, &size, &type);
+ if (type != MTRR_TYPE_WRBACK)
+ continue;
+ if (highest_pfn < base + size)
+ highest_pfn = base + size;
+ }
+
+ /* kvm/qemu doesn't have mtrr set right, don't trim them all */
+ if (!highest_pfn) {
+ if (!kvm_para_available()) {
+ printk(KERN_WARNING
+ "WARNING: strange, CPU MTRRs all blank?\n");
+ WARN_ON(1);
+ }
+ return 0;
+ }
+
+ if (highest_pfn < end_pfn) {
+ printk(KERN_WARNING "WARNING: BIOS bug: CPU MTRRs don't cover"
+ " all of memory, losing %luMB of RAM.\n",
+ (end_pfn - highest_pfn) >> (20 - PAGE_SHIFT));
+
+ WARN_ON(1);
+
+ printk(KERN_INFO "update e820 for mtrr\n");
+ trim_start = highest_pfn;
+ trim_start <<= PAGE_SHIFT;
+ trim_size = end_pfn;
+ trim_size <<= PAGE_SHIFT;
+ trim_size -= trim_start;
+ update_memory_range(trim_start, trim_size, E820_RAM,
+ E820_RESERVED);
+ update_e820();
+ return 1;
+ }
+
+ return 0;
+}